1. Field of the Invention
The present invention relates to an arrangement of thin film transistors (TFTs) each having a polycrystalline semiconductor channel in a substrate for a display device having active elements (i.e., an active matrix display element, a thin film active element substrate or a TFT substrate), a TFT display element using the substrate and a method of making them.
2. Discussion of Background
Recently, there has been a strong demand for flat panel displays for display devices instead of using CRTs. Among the flat panel displays, a liquid crystal display element (LCD) is considered to be most desirable. To meet demands of a color display and a high speed display, active matrix type LCDs utilizing TFTs are commercialized.
Generally, amorphous silicon (.alpha.-Si) is used for a semiconductor layer for a TFT. However, use of polycrystalline Si instead of .alpha.-Si allows to form a small-sized TFT, to obtain a high speed operation and to form a liquid crystal display device having a large picture displaying surface area and a high density because the mobility is high. Further, it is possible to simultaneously form driver circuits and TFTs for display device on a single substrate.
A method of forming a polycrystalline Si layer on a quartz substrate at a high temperature such as about 1000.degree. C. or a method of forming the polycrystalline Si layer on a glass substrate at a low temperature such as 600.degree. C. or lower have been known. In addition, there is known a method of obtaining polycrystalline Si by irradiating a laser beam spot to .alpha.-Si and beam-annealing .alpha.-Si under the condition of about room temperature.
In order to form a liquid crystal display element having a large picture area and a high performance with a high productivity, it is desirable to form a polycrystalline Si layer on an ordinary glass substrate for LCD (e.g. Corning 7059). In addition, a process of low temperature such as 600.degree. C. or lower is needed. The above-mentioned beam-annealing method is desirable to achieve such requirements.
For the method of forming polycrystalline Si by beam-annealing, there is a first method wherein the entire surface area of a substrate or a portion of the substrate required to form a polycrystalline Si layer is beam-annealed without remaining non-annealed portion, and a second method wherein a portion unnecessary for the beam-annealing is omitted. In the former method, a laser beam excited by pulse oscillation and having a large irradiation surface area, such as an excimer laser, is widely used. In the later method, a laser beam of continuous oscillation such as an argon ion laser is widely used. The later method is preferable in a case that a high speed treatment is required and the throughput has to be improved.
There are statements concerning polycrystalline Si TFT liquid crystal panels in Nikkei Electronics, vol. 602 (Feb. 28, 1994), P. 103-109. The following is the summery of a statement described in P. 106-P. 107.
"Now, it is difficult for the beam anneal (laser beam anneal) method to uniformly control the mobility of the polycrystalline Si in the plane. Further, at present, there is no large aperture type laser device which can irradiate a substrate having a large surface area at once. Therefore, the large substrate is annealed by the scanning of a pulse laser having an aperture diameter of about 5 mm-10 mm. In this case, there is a problem that an overlapping portion is resulted in the laser treatment, and accordingly, there is an in-plane variation of about .+-.50% in terms of the mobility, which results in the variation of the characteristics of the TFTs.
As measures to the above problem, the speed of treatment is reduced to minimize the overlapping portion of polycrystalline Si. Or, variation in test products could be suppressed to about .+-.10% by conducting a plurality of laser treatments. A further problem for a large scale production, which has to be considered, is to increase the treating speed while ununiformity is controlled."
As a technique in the beam-annealing method, there is a high speed beam-annealing method using laser beam (HSBA) which is featurized by scanning beam spots at a high speed. For instance, the scanning is conducted with a laser output power of about 7W-25W and a linear speed for scanning of 10 m/s-20 m/s, preferably, 10 m/s-15 m/s, more preferably 11 m/s-13 m/s. As prior art techniques for forming polycrystalline Si TFTs by using the HSBA, there are those described in Japanese Unexamined Patent Publication No. 226039/1992 and Japanese Unexamined Patent Publication No. 226040/1992, (these applications are corresponding to U.S. Pat. No. 5,306,651). According to the HSBA, it is possible to polycrystallizing .alpha.-Si at a process temperature of 450.degree. C. or lower.
In the HSBA method, only portions forming silicon islands which become polycrystalline semiconductor active layers for TFTs are annealed by irradiating the laser beam. The remaining portions,used for wiring and pixel electrodes are not subjected to the beam-annealing. For instance, when a TFT for displaying having polycrystalline Si is formed on a substrate with use of a scanning type beam annealing device, the scanning of the laser beam is conducted for irradiation at the same number of times as the number of the row electrode lines of a matrix which forms a picture surface.
As another conventional technique, there is Japanese Examined Patent Publication No. 9794/1993, which is featurized by laser-annealing only a non-single crystalline Si TFT in a peripheral driving circuit other than the pixel area of a LCD. For instance, a CW exciting YAG laser is used as a light source, and a light beam having a beam diameter of 200 .mu.m is scanned at a linear speed of 50 cm/sec in the left and right directions so that only the portion of the peripheral driving circuit is processed by laser annealing.
In the above-mentioned conventional technique, there is description that the laser annealing to the entire surface of the substrate is not practical because the throughput in the manufacturing is very poor (due to a low linear speed). Accordingly, only the peripheral driving circuit is polycrystallized to form a polycrystalline semiconductor transistor. On the other hand, .alpha.-Si is used for the pixel area. There is further description that when a peripheral circuit for a row electrode line and a peripheral circuit for a column electrode line are annealed, the substrate can be turned for the laser annealing. Further, there is statement on the performance of the LCD that the number of the both column electrode lines (data lines) and the row electrode lines (gate lines) are 200, and a polycrystalline semiconductor layer having a sufficient mobility and an operating speed required for the circuits have been able to attain.
In the next, the positional relation of the Si islands in the beam-annealing method will be described. FIG. 2 is a plan view showing a relation of the arrangement of Si islands 1 to polycrystallized stripes 2 in column driving circuits wherein the Si islands are not at sufficiently regulated positions. In order to anneal all the Si islands 1, it is necessary to conduct beam-anneal-scanning without remaining portions of scanning, which increases the number of times of scanning. As a result, it takes much time to conduct the beam-annealing of TFTs for column driving circuits, resulting in the reduction of the throughput.
In a case of making the above-mentioned substrate for a display device by using the beam-annealing method, it was difficult to beam-anneal TFTs for row driving circuits or column driving circuits which are provided at peripheral portions other than a region for pixels at the same time of the beam-annealing of TFTs for driving the pixels.
When the arrangement of the TFTs in the circuits at the peripheral portions of the substrate is not adequately made, it takes much time to beam-anneal these portions, whereby the throughput is reduced.